1. Field of the Invention
This invention relates to a carrying belt driving apparatus for carrying sheet-shaped recording material with a carrying belt and, more particularly to a carrying belt driving apparatus for successively carrying recording material to a plurality of image recording units and an image recording apparatus using the same.
2. Related Background Art
In a conventional carrying belt apparatus, sheet-shaped transfer material such as ordinary paper is electrostatically attracted to the carrying belt for carrying. In a conventional color printer apparatus, in which a color image based on multiple transfer image is formed on sheet-shaped transfer material by allowing a plurality of transfer units in image forming means to successively pass, the sheet-shaped transfer material is carried with the carrying belt for multiple transfer in each transfer unit. Therefore, an image displacement is greatly controlled by consistency of movement of the carrying belt, and after all, a high roundness is required for a driving roller to drive the carrying belt, and at the same time, a high rotational speed is also required for a gear train which interlocks the driving roller.
For this reason, such an apparatus as shown in FIG. 1 has been proposed so far (prior application by Canon Inc., Japanese Patent Laid-Open Application No. 59-182139 official gazette).
This conventional apparatus comprises four sets of electrophotographic laser beam printer mechanism included as a plurality of sets of image forming mechanisms. That is, in FIG. 1, numeral 1 is a main body box of the apparatus, and numerals I, II, III and IV show four sets: the 1st to the 4th of laser beam printer mechanisms (hereinafter called simply "printer mechanism") which have been successively disposed from the right to the left in FIG. 1 within the main body box 1.
Belt driving rollers 3 and 4 have been disposed diagonally to the lower right of the 1st printer mechanism, and diagonally to the lower left of the 4th printer mechanism IV respectively, and are driven by a driving source (not shown). A screen belt 2 for carrying the transfer material is wound around the belt driving rollers 3 and 4. This screen belt 2 is made of Tetron fiber mesh, and is moved in the arrow direction shown in FIG. 1 by the driving rollers 3 and 4. A paper feed mechanism 5 has been disposed on the right side of the apparatus frame, and an image fixer 6 on the left end side thereof. Numeral 7 is a discharge port outside the printer.
Each printer mechanism I to IV are substantially the same in mechanism constitution itself. That is, each printer mechanism is composed of a drum type electrophotographic photosensitive body 9 (hereinafter simply called "drum") as an image bearing body which is driven around a shaft 8 in the arrow direction, a charger 10, a developer 11, a transfer charger 12 and a cleaner 13 which have been successively disposed around the drum 9 in the direction of rotation thereof, a laser beam scanner 14 disposed above the drum 9, and the like.
The laser beam scanner 14 is composed of a semiconductor laser, a polygon mirror, an f-.theta. lens, a light shielding plate, etc., and receives the input of an electric digital pixel signal S of time series to be calculated and output by an image reading apparatus (not shown) and an electronic computer to oscillate a laser beam L modulated in accordance with the signal, and to expose the drum surface by scanning a drum surface portion between the charger 10 and the developer 11 in the drum generatrix direction.
However, yellow (Y) developing toner is kept in a developer 11 of the first printer mechanism I, magenta (M) developing toner is kept in that of the second printer mechanism, cyan (C) developing toner in that of the third printer mechanism, and black (BK) developing toner in that of the fourth printer mechanism respectively.
A pixel signal S (Y) corresponding to a yellow component image of color image is input into a laser beam scanner 14 of the 1st printer mechanism I, a signal S (M) corresponding to a magenta component image is input into that of the 2nd printer mechanism II, a signal S (C) corresponding to a cyan component 10 image into that of the 3rd printer mechanism III, and a signal S (BK) corresponding to a black component image into that of the 4th printer mechanism IV respectively.
When power is turned on for the apparatus, current is flown through the laser beam scanners 14 for each printer mechanism I to IV and other required process equipment or these are driven, and current is flown through the heater for a fixer 6 to cause the apparatus to perform warming-up operation. When the laser lights, the scanner reaches a predetermined number of revolutions, and the fixing roller reaches a predetermined temperature, this printer apparatus is ready for operation.
When cut sheet-shaped transfer sheet P as transfer material is inserted on a paper feed guide 51 of the paper feed mechanism 5, its tip end is detected by a first photointerrupter 52 to transmit a start signal (start signal of print sequence). This start signal starts to rotate the drum, 9 for each printer mechanism I to IV. The driving rollers 3 and 4 are also driven at the same time to start running the screen belt 2 in the arrow direction.
The transfer sheet P is fed on the screen belt 2 through paired registers 53, a paper feed guide 55, paired registers 56 and a paper feed guide 57. The transfer sheet P on the screen belt 2 receives corona discharge from a charger for attraction 59 to be securely attracted to the screen belt 2. A guide 58, a conductor, is provided as a counter electrode at this charger 59, and this counter electrode 58 is specially effective if grounded.
Further when the tip end of the transfer sheet P interrupts each photointerrupter 60Y, 60M, 60C and 60BK on the downstream side, its signal successively starts forming of an image for each drum 9, which has been rotating beforehand, of each printer mechanism I to IV.
That is, an yellow image as color component of color image is assigned to the drum 9 surface of the 1st printer mechanism, the same magenta image to that of the 2nd mechanism II, the same cyan image to that of the 3rd mechanism III, and the same black image to that of the 4th mechanism IV for being formed respectively. Since the principle of forming an image in each printer mechanism has been already known well as Carlson process, its description is omitted.
Rotation of the screen belt 2 allows the transfer sheet P to successively pass the lower portion of the 1st to 4th printer mechanisms I to IV toward the fixer 6 for being carried. In the process of passage in each mechanism unit, a yellow image formed on the drum 11 surface of the 1st printer mechanism I, the same magenta image on that of the 2nd mechanism II, the same cyan image on that of the 3rd mechanism III and the same black image on that of the 4th mechanism IV are successively piled up and transferred on the surface of the transfer sheet by a charger for transfer 12 of each mechanism unit to synthetically form a color image on the surface of the sheet. After the transfer sheet passes the 4th printer mechanism IV, it is de-electrified by a deelectrifier 61, to which AC voltage has been applied, and is separated from the screen belt 2 without causing a discharge mechanism.
The transfer sheet P gets on a separating pawl 61a, enter the fixer 6, the image is fixed through color toner formed thereon, and the sheet is discharged outside the apparatus through an outlet 7 as a color image print. After the transfer sheet P is discharged outside the apparatus, all rotations except the fixer are stopped to complete one print cycle.
To detect the tip end of the above-mentioned transfer sheet P, each photointerrupter 60Y, 60M, 60C and 60BK has been disposed between each mechanism on the path of movement of the screen belt 2 toward the 1st to 4th printer mechanism I to IV on the upstream side of each transfer unit, and plays a role to determine a timing of starting image formation for each mechanism by detecting successive passage of the transfer sheet P through each mechanism unit. Tension rollers 62 and 63 give a tension to the screen belt 2, and the tension roller 62 is rotatable, but its position is fixed. On the other hand, the tension roller 63 is rotatable and also rockable in the arrow direction.
In such a conventional apparatus, the screen belt 2 is driven with a frictional force by the driving roller 4. This driving roller 4 is constructed so that its circumference is equal to an interval between each transfer station (a distance of the screen belt between each transfer station) H. That is, assuming the diameter of the driving roller 4 to be D, it has a relationship of .pi.D=H.
If the driving roller 4 has an eccentricity, etc. due to processing, assembly, and the like in this case, the moving speed of the screen belt 2 does not become constant, but changes like a sine curve as shown in FIG. 2. According to the constitution of the above prior art, however, the period T.sub.1 of the above-mentioned sine wave coincides with a duration in which the transfer sheet P on the screen belt 2 moves from one transfer station to the next transfer station. The amounts of expansion and shrinkage of an image transferred by two transfer stations (2nd and 3rd printer mechanisms II and III) at this time are plotted as shown in FIG. 3.
That is, the transfer position of the image changes like a sine curve as compared with the ideal transfer position, but the phase of the sine wave of an image to be transferred also coincides always because the phase angle of the eccentricity of the driving roller 4 at a position of starting the transfer for each color is always fixed. For this reason, no relative color drift for each color on the image occurs.
However, it has become necessary in recent years to shorten a distance between each image recording unit with miniaturization and weight reduction of a printer apparatus. As a result, in such a conventional belt driving apparatus as mentioned above, the diameter of the belt driving roller must be also made smaller when the distance between the image forming stations for each color is shortened. Assuming the distance between each station to be, for example, 20 mm, the diameter of the driving roller becomes about 6.3 mm. Since, however, the contact area between the driving roller and the belt is considerably small in this case, a problem in which the belt cannot be surely carried, and further a problem in which the belt driving apparatus has an insufficient strength because the shaft diameter is small have occurred.